The present invention relates generally to mobile radio systems and more particularly to systems using the code division multiple access (CDMA) technique.
The CDMA technique is used in third generation systems such as the Universal Mobile Telecommunication System (UMTS), for example.
As a general rule, a mobile radio network includes base stations and base station controllers, as shown in FIG. 1. In the UMTS, the network is known as the UMTS Terrestrial Radio Access Network (UTRAN), a base station is known as a Node B, and a base station controller is known as a Radio Network Controller (RNC).
A mobile station is known as a User Equipment (UE), and the UTRAN communicates with mobile stations via a Uu interface and with a Core Network (CN) via an Iu interface.
As shown in FIG. 1, an RNC is connected:                to a Node B via an Iub interface,        to other RNC via an Iur interface, and        to the core network (CN) via an Iu interface.        
The RNC controlling a given Node B is known as the Controlling Radio Network Controller (CRNC) and is connected to the Node B via the Iub interface. The CRNC has a load control function and a radio resource allocation and control function for each Node B that it controls.
For a given call relating to a given user equipment UE, there is a Serving Radio Network Controller (SRNC) that is connected to the core network via the Iu interface. The SRNC has a control function for the call concerned, including the functions of adding or removing radio links in accordance with the macrodiversity transmission technique, monitoring parameters likely to change during a call, such as bit rate, power, spreading factor, etc.
In CDMA systems, capacity limitations at the radio interface are fundamentally different from their counterparts in systems using other multiple access techniques, such as the Time Division Multiple Access (TDMA) technique. The TDMA technique is used in second generation systems such as the Global System for Mobile communications (GSM), for example. In CDMA systems, at any time all users share the same frequency resource. The capacity of these systems is therefore limited by interference, for which reason these systems are also known as soft limited systems.
This is why CDMA systems use algorithms such as load control algorithms to prevent, detect, and where applicable correct overloads, in order to avoid degraded quality, and call admission control algorithms to decide (as a function of diverse parameters such as the service required for the call, etc.) if the capacity of a cell that is not being used at a given time is sufficient for a new call to be accepted in that cell. In the remainder of the description, these algorithms are grouped together under the generic name load control algorithms.
They ordinarily use only radio criteria and are ordinarily executed in the CRNC, which does not have any information on the processing capacity of each Node B that it controls. It can therefore happen that the CRNC accepts a new call only for the call to be finally rejected because of a shortage of processing resources in the Node B, which leads to unnecessary additional processing in the CRNC and additional exchanges of signaling between the CRNC and the Node B.
Of course, it would be possible to avoid these problems by providing each Node B with sufficient processing resources to cover all situations, including that of maximum capacity (which corresponds to the situation of a very low level of interference). However, this would lead to costly base stations that would be rated more highly than necessary most of the time. Furthermore, in the case of progressive introduction of services offered by these systems, the processing capacity of the base stations can be limited at the start of deployment of these systems and progressively increased thereafter.
It would therefore be desirable for load control in such systems to allow for the processing capacity of each base station (Node B).
FIGS. 2 and 3 respectively outline the main transmit and receive processing used in a base station, for example a UMTS Node B.
FIG. 2 shows a transmitter 1 including:                channel coding means 2,        despreading means 3, and        radio frequency transmitter means 4.        
These processing means are well known to the person skilled in the art and do not need to be described in detail here.
Channel coding uses techniques such as error corrector coding and interleaving to protect against transmission errors. This is also well known to the person skilled in the art.
Coding (such as error corrector coding) is intended to introduce redundancy into the information transmitted. The coding rate is defined as the ratio of the number of information bits to be transmitted to the number of bits actually transmitted or coded. Various quality of service levels can be obtained using different types of error corrector code. In the UMTS, for example, a first type of error corrector code consisting of a turbo code is used for a first type of traffic (such as high bit rate data traffic), while a second type of error corrector code consisting of a convolutional code is used for a second type of traffic (such as low bit rate data or voice traffic).
Channel coding generally also includes bit rate adaptation in order to adapt the bit rate to be transmitted to the bit rate offered for its transmission. Bit rate adaptation can include techniques such as repetition and/or puncturing, the bit rate adaptation rate then being defined as the repetition rate and/or the punch-through rate.
The raw bit rate is defined as the bit rate actually transmitted at the radio interface. The net bit rate is the bit rate obtained after deducting from the raw bit rate everything that is of no utility to the user, for example the redundancy introduced by the coding process.
Spreading uses spectrum spreading principles that are well known to the person skilled in the art. The length of the spreading code used is known as the spreading factor.
It should not be forgotten that, in a system such as the UMTS, the net bit rate (which is referred to hereinafter for simplicity as the bit rate) can vary during a call, and that the spreading factor can vary as a function of the bit rate to be transmitted.
FIG. 3 shows a receiver 5 including:                radio frequency receiver means 6,        received data estimation means 7, including despreader means 8 and channel decoder means 9.        
These processing means are well known to the person skilled in the art and therefore do not need to be described in detail here.
FIG. 3 shows an example of processing carried out in the despreader means 8. The processing is carried out in a rake receiver to improve the quality of received data estimation using multipath phenomena, i.e. propagation of the same source signal along multiple paths, such as results from multiple reflections from elements in the environment, for example. Unlike TDMA systems, CDMA systems can exploit the multiple paths to improve received data estimation quality.
A rake receiver has a set of L fingers 101 to 10L and means 11 for combining signals from the fingers. Each finger despreads the signal received via one of the paths that are taken into account, as determined by means 12 for estimating the impulse response of the transmission channel. To optimize received data estimation quality, the means 11 combine the despread signals corresponding to the paths that are taken into account.
The reception technique using a rake receiver is also used in conjunction with the macrodiversity transmission technique, whereby the same source signal is transmitted simultaneously to the same mobile station by a plurality of base stations. By using a rake receiver, the macrodiversity transmission technique not only improves receive performance but also minimizes the risk of call loss during handover. For this reason it is also known as soft handover, as compared to hard handover, in which a mobile station is connected to only one base station at any given time.
The received data estimating means can also use various techniques for reducing interference, such as the multi-user detection technique.
It is also possible to use a plurality of receive antennas. To optimize received data estimation quality the received data estimator means then further include means for combining signals received via the multiple receive antennas.
Channel decoding includes functions such as deinterleaving and error corrector decoding. Error corrector decoding is generally much more complex than error corrector coding and can use techniques such as maximum likelihood decoding, for example. A Viterbi algorithm can be used for convolutional codes, for example.
To be able to process several users simultaneously, a base station (Node B) includes transmitters and receivers such as the transmitter and the receiver outlined above and therefore requires a high receive processing capacity for received data estimation.
As mentioned above, for monitoring the load in a system such as the UMTS, for example, it is therefore desirable to take account of the processing capacity of a base station.
For the UMTS, for example, the 3G document TS 25.433 published by the 3rd Generation Partnership Project (3GPP) specifies that, for each value of the spreading factor (SF) for which there is provision within the system, the Node B must signal to the CRNC its overall processing capacity (also known as the capacity credit) and the amount of that overall processing capacity (also known as the consumption cost) that is necessary for allocating a physical channel. The set of all consumption costs for all possible values of the spreading factor is also known as the capacity consumption law. This information is signaled by a Node B to the CRNC each time that the processing capacity of the Node B changes, using a Resource Status Indication message, or in response to a request from the CRNC, using an Audit Response message.
French Patent Application No. 0010538 filed Aug. 10, 2000 by the Applicant states that this kind of solution is not suitable for taking account of processing capacity limitations in a Node B, for the following reasons:                The channel decoding processing depends on the net bit rate rather than the gross bit rate or the spreading factor. For example, considering a spreading factor of 128 (and thus a raw bit rate of 30 kbit/s), the net bit rate can have different values depending on the coding rate and the bit rate adaptation rate, and the net bit rate can typically vary from 5 to 15 kilobits per second (kbit/s). Consequently, for a fixed spreading factor, the amount of processing in the Node B can vary significantly (for example by a factor exceeding 3). This is not taken into account in the prior art solution.        The number of fingers of the rake receiver required for transmission channel and data estimation is highly dependent on the number of a radio links. In the prior art solution, the maximum number of fingers of the rake receiver in the Node B cannot be taken into account in algorithms such as load control algorithms or call admission control algorithms, as this type of limitation is not linked to the spreading factor.        The processing capacity signaled by the Node B to the CRNC is an overall processing capacity that cannot take account of limitations in the processing capacity of the Node B.        
The earlier patent application proposes another approach whereby, to take account of limitations in the processing capacity of a Node B, the Node B signals to the CRNC one or more parameters such as the maximum number of radio links that can be set up and the maximum net bit rate for the radio links that have been set up, possibly for each transmission direction and/or for each type of channel coding that can be used.
The present invention provides a new approach which retains the concept of overall processing capacity (also known as the capacity credit) but in which the consumption cost is no longer signaled for each possible value of the spreading factor, but instead for possible values of the bit rate (as already indicated hereinabove, the Applicant has noted that the bit rate is more representative of the processing capacity of a Node B than the spreading factor).
However, anew approach of this kind implies that new problems need to be solved.
A first problem is that, although the number of possible spreading factors is finite (for example, there are eight possible spreading factors in the UMTS: 4, 8, 16, 32, 64, 128, 256, 512), the bit rate can take any positive value. It is clear that in practice it is not possible or realistic for the Node B to signal the consumption cost to the CRNC for all bit rate values.
A second problem is that, at least under the present standard, the CRNC does not know the bit rate in order to update the capacity credit on each allocation of resources as a function of the corresponding bit rate. However, in the above-outlined prior art solution, the CRNC knows the spreading factor because the SRNC signals the spreading factor to the CRNC if a new radio link is added, removed, or reconfigured.
A third problem is that the bit rate need not be fixed, but may vary. However, the spreading factor is fixed, at least for the downlink direction, and is signaled in the manner mentioned above. The spreading factor for the uplink direction also varies, but the Applicant has noted that this aspect of the problem is not taken into consideration in the above-outlined prior art solution using the spreading factor.